1. Field of the Invention
The present invention relates to a rotor blade structure for a vertical shaft impact crusher. More particularly, the present invention relates to a rotor blade structure for a vertical shaft impact crusher for crushing bulk materials, for example, natural rock, into particles of predetermined diameter.
2. Description of the Background Art
Bulk materials, e.g., natural rock, are crushed in accordance with various uses, for example, aggregate for concrete, paving stone, subgrade material, etc. One type of crusher used for such crushing process is known as impact crusher.
Impact crushers operate on the basis of the principle that rock is accelerated at high speed so as to collide with an impact surface, thereby crushing the rock. Such impact crushers may be roughly divided into two types according to the mode of crushing: anvil type and dead stock type.
In the anvil type impact crusher, a rotor having a plurality of blades on the upper side thereof is rotated at high speed, whereby raw stone cast in the crusher is accelerated by the blades and centrifugally discharged so as to collide with anvils which are disposed in a ring shape around the rotor, thereby crushing the raw stone. The anvil type impact crusher is mainly used for the purpose of crushing raw stone having a relatively large diameter by collision to thereby reduce the size of the raw stone.
On the other hand, the dead stock type impact crusher is used to smooth the surfaces of particles of raw stone which has already been crushed into gravel of desired size and to make the particle size uniform. That is, the dead stock type impact crusher is similar to the anvil type impact crusher in that raw stone is accelerated by blades, but different from the latter in that a dead stock is formed from crushed raw stone at the periphery of the rotor, and this dead stock is used as an impact surface for crushing raw stone.
In either of the anvil type and dead stock type impact crushers, as the service time of the crusher elapses, the blades are worn out. For this reason, manganese steel or other wear-resistant material is used for the blades. However, even if such a material is used, wear of the blades cannot be avoided, and the blades must be frequently replaced with new ones. Various propositions have heretofore been made to reduce the frequency of replacement of the blades and to thereby facilitate maintenance.
For example, Japanese Patent Application Public Disclosure (KOKAI) No. 62-193657 (1987) discloses a vertical shaft impact crusher in which a pair of blades, which define a discharge passage, are provided on the upper side of a rotor in symmetry with respect to the radial direction, and the rotor is rotated forwardly and then backwardly, thereby avoiding non-uniform wear of the blades. With this crusher, the range of collision between raw stone and the blades is enlarged by reversing the direction of rotation of the rotor, so that non-uniform wear can be prevented to a certain extent. Therefore, the frequency of replacement of the blades also decreases. However, it does not mean that the lifetime of each individual blade increases, even if the impact crusher is arranged as described above.
A great variety of impact crushers have heretofore been known in which the blades are curved to form a dead stock from crushed rock on the rotor along the side surfaces of the blades so that the blades are protected from wear by the dead stock (for example, see Japanese Utility Model Application Public Disclosure (KOKAI) Nos. 64-56834 (1989) and 01-163539 (1989)). In this case, since no dead stock is formed at the outer peripheral ends of the blades, a carbide tip is provided on the outer peripheral end of each blade with the surface of the carbide tip exposed with a view to avoiding wear of the blade. In this type of impact crusher, in which the blades are protected by the dead stock, the lifetime of the blades increases to a certain extent by virtue of the presence of the carbide tip. However, when a blade has become worn in excess of a predetermined level, the whole blade must be replaced with a new one, which is a waste of the blade material.
Although the carbide tip is not readily worn out, raw stone directly collides with the carbide tip from the beginning of use of the crusher because it is exposed at the outer peripheral end of each blade. Accordingly, the lifetime of the blades does not increase so much as is expected.